A reservoir is formed of one or more subsurface rock formations containing oil and/or gas. The reservoir rock is porous and permeable. The degree of porosity relates to the volume of liquid and gas contained in the reservoir. The permeability relates to the reservoir fluid's ability to move through the rock and be recovered for production to the surface.
Reservoirs are conventionally drilled overbalanced in which drilling fluid, chemicals and weighted material are circulated into and out of the well bore to maintain well control by making hydrostatic pressure in the well bore exceed the reservoir's pressure. A disadvantage in drilling overbalanced is formation damage that occurs when solids, fines and chemicals that are circulated into the well bore to maintain the overbalanced condition penetrate into the formation. Such penetration, or skin damage, impedes or stops hydrocarbons that would otherwise flow into the well during production operations.
Reservoirs may also be drilled at balance, near balance or under balanced. For under balanced drilling, drilling fluids with lower specific gravities are used such that hydrostatic pressure in the well bore is less than the reservoir's pressure. This allows hydrocarbons to flow into the wellbore when the formation is drilled. The hydrocarbon inflow reduces or prevents entry of drilling fluids and associated contaminants into the formation and thereby reduces formation skin damage. Higher production rates typically result from under balanced drilling.
During or after drilling, reservoirs are evaluated to determine the type, amount and value of contained hydrocarbons. The characteristics of a reservoir may be extrapolated from the small portion of a formation exposed during the drilling process. For example, data may be collected from drilling, well testing, logging and coring operations.
Reservoir evaluation may be aided by underbalance drilling in which reservoir fluids are allowed to flow into the well during drilling. Drilling fluid, gas and formation samples may be taken for laboratory analysis. Such analysis is typically performed after the completion of drilling operations. For example, the analysis may be performed during production logging or testing operations.
On site reservoir evaluation systems have also been used for under balanced drilling. Such systems provide mud logging and basic compositional analysis data. This data, along with pressure and rate data, has been used in connection with well bore modeling to determine reservoir permeability and other productivity information during drilling. Such information, however, is typically not available for wells drilled overbalanced or wells that oscillate between over and under balanced.